مجله کیفیت و بهره وری صنعت برق ایران
سال دوازدهم شماره 1 (پیاپی 30، بهار 1402)

Power transformers are the most important components of a power system, so their protection is a critical issue. This paper proposes a novel and efficient algorithm based on the high-frequency components of the differential current signal to discriminate between the magnetizing inrush currents and the internal faults. After detecting the over-current in the differential current signals, samples of a quarter of a cycle of the signal are recorded. Then, discrete wavelet transform (DWT) is applied to the recorded signals, and the details of the wavelet transform output are extracted. Because of the existence of the high-frequency transients in the internal fault current signals, the wavelet transform outputs of the internal fault signals have more fluctuations than that of the inrush current signals. By calculating the standard deviation of the wavelet transform output, the fluctuations can be quantified. Therefore, the standard deviation of the wavelet transform output can be used as a criterion to discriminate between the internal faults and the magnetizing inrush currents. The proposed algorithm has a very low computational burden, and it uses only a quarter of a cycle of the differential current signals. This guarantees the high speed of the proposed algorithm. The proposed algorithm is tested by different conditions of the internal faults and the inrush situations, and it successfully identifies the true situation with high accuracy in all conditions. The simulation results show the superior specifications of the proposed algorithm.

Nowadays, renewable energy is increasingly used in smart grids and microgrids to reduce the use of fossil fuels and improve network efficiency. Like all power system devices, microgrids are subject to transient and steady-state faults, such as short circuits. These faults impair reliability and consumer dissatisfaction. To accurately, automatically, and economically determine the location of a fault, a robust fault location method is needed to stabilize and repair the damaged part of the network. Given the access to the data of all nodes, the fault in these networks can be located based on the data on the two terminals. Accordingly, this paper proposes a method for determining fault distance and faulty section in the island and grid-connected microgrids. The proposed method uses distributed parameters line model and calculates the location of double-phase faults in the microgrid based on voltage and current data on both sides of each section, taking renewable energies and electric vehicles into account. At first, the measurement devices receive and store the current and voltage data at the beginning and end of each section. If a fault occurs, the fault distance is determined by calculating the difference between voltages and currents on both sides of the fault. According to the sampling rate, many voltage and current samples are obtained during the fault. The proposed method calculates a fault distance for each sample. As a result, many fault distances are obtained. These calculations are done for all sections. In the next step, the distances obtained for each section are plotted on the coordinate axis, and a curve is obtained for each section. Among the curves obtained, one curve has a global minimum, which indicates the faulty section. Other curves are ascending or descending. In addition, the global minimum point indicates the calculated distance of the fault from the beginning of the section. This method is not sensitive to electric vehicle models and distributed generation sources and uses only less than half-cycle data to execute the algorithm. The performance of the method is investigated with the simulation of a 9-bus microgrid in MATLAB/SIMULINK. The effects of changes in line parameters (two scenarios), different fault locations, fault resistance (0, 25, and 50 Ω), fault inception angles (36, 90, 180, and 270 degrees), different DGs operation modes (three scenarios), and measurements error (±3%) are studied. The maximum and minimum errors of this method are obtained to be 0.97% and 0.02%, respectively. The results indicate the high accuracy of the proposed method compared to other fault location methods.

The analysis of transient stability in electric power systems and the penetration coefficient of scattered productions play important roles in the regulation of protective equipment, so these parameters should be considered two main factors in the electrical protection of power networks. Overcurrent relays are used as one of the simplest and most effective solutions for power system protection. The overcurrent relay has two main variables: time setting factor (TMS) and plug setting (PS). The relay operation time is a function of TMS, PS, and the current seen by the relay. This paper analyzes the transient stability of a distribution network including distributed generation to determine the protective regulation of overcurrent relays. First, the transient stability of scattered products is studied for different fault locations and their different penetration coefficients. Then, the limitations of distributed generation resources are considered in the protection coordination of overcurrent relays. Therefore, a new method is proposed to calculate the modified value of the time adjustment coefficient of the overcurrent relay. In other words, not only the protection limits but also the transient stability limits are considered in the calculation of the time adjustment factor. In fact, by applying the proposed method, not only is the coordination between overcurrent relays maintained but the instability of distributed generation sources is also prevented. The performance of the proposed protection plan is evaluated with the help of simulation in an IEEE 33-bus distribution system using ETAP software. The results show the correct performance of the proposed method for the worst error conditions. It should be noted that to carry out conservation and stability studies, the following steps are performed in order, which ultimately leads to the creation of a conservation-stability algorithm: (A) protection studies in the condition of disconnection of DG resources, (B) transient stability studies in the conditions of connection of scattered production sources, including obtaining the minimum stability time for each of the scattered production, and (C) investigating the simultaneous establishment of CTI and CCT. In this situation, two situations are possible: the simultaneous establishment of CTI and CCT, the establishment of CTI restrictions, and the non-establishment of CCT. The value of CCT for each DG is equal to the time when the speed of the DG or the power angle of its generator becomes zero. As can be seen, in the first, second, fourth, and fifth scenarios, the CCT condition of the distributed generation source is not established, and it is necessary to modify the TDS value of the main relay.

Reconfiguration of distribution network feeders is one of the well-known and effective strategies in the distribution network to obtain a new optimal configuration for the distribution feeders by managing the status of switches in the distribution network. This study formulates the multi-objective problem of reconfiguration of a distribution network in the optimal presence of distributed generation sources and capacitor units in a multi-objective format. Also, the effect of uncertainty related to electric charge is included in the optimization process of the problem.  The optimization problem of the distribution network reconfiguration is non-linear and non-convex, considering that the effect of distributed and capacitive generation units makes the optimization problem more complicated. For this purpose, an improved gray wolf optimization algorithm is presented to solve this optimization problem. Then, the values ​​of the objective functions are normalized using fuzzy membership functions, and finally, fuzzy logic is used to find the most optimal solution among the Pareto solutions. To verify the effectiveness of the proposed method, it is tested on a test system of 33 pools, and the results of the optimization are compared with those of other evolutionary algorithms.

This paper presents a model for distribution network optimization considering a high penetration of photovoltaic (PV) sources and electric vehicle charging stations (EVCSs) based on on-load tap changing transformers (OLTC) and step voltage regulator (SVR), shunt capacitor (SC), and shunt reactor (ShR). The purpose is to prevent overvoltage due to power injection by PV sources and voltage drop due to EV charging in distribution networks. The proposed model is solved using a new hybrid algorithm called PSO-GA. Relevant studies show that with the increasing number of PSO replications, particle population variability is easily eliminated and placed in local optimization. The idea of ​​combining GA is based on the PSO introduced in this study. Crossover and mutations of GA are performed on the PSO population, which is useful for improving the overall optimal ability of particles and causing the algorithm to deviate from the local optimal point. Two different IEEE standard test networks are tested under different load scenarios to analyze the proposed model. The results reveal the performance of the proposed model.

Nowadays, the clean, non-pollutant, free and renewable energy sources are in the center of attention in industry to meet the electric energy requirements. The smart energy networks need the energy storage systems as interfaces to realize the energy management and balance between the generated and demanded energy. The power electronic converters can efficiently play the role of interface between the energy sources, load and energy storage systems. The solar energy is one of the popular renewable energies that can be converted to the electric energy through the Photovoltaic (PV) panels. The output voltage (energy) of PV panels is usually low and in DC form. Thus, the DC-DC power converters are required to enhance the low output voltage of PVs to higher practical values. They should also be capable of extracting and tracking the maximum power point of PVs. Different topologies have been presented in the literature that fulfill some requirements of DC-DC converters, such as: step-up capability, availability of common ground point, continuous source current, low required devices, low current and voltage stress on the components and so on. This paper proposes an improved configuration that combines the impedance network and quadratic boost converter, which produces high voltage boosting factors (even at low or mild duty ratios) compared to its similar counterparts. Moreover, the voltage stress of its switching devices is limited to output voltage. It also benefits from continuous input current and common-ground point, which makes it very suitable for solar cell applications. The two switches of proposed converter operate synchronously, which leads to only two operational states in continuous conduction mode (CCM) and easy control strategy. The superiority of the proposed configuration over its counterparts has been certified by comprehensive comparative analysis. Furthermore, the real and ideal voltage gain of the proposed converter has been calculated and presented. The dynamic behavior of proposed converter in the closed loop control system has been investigated through the simulation analysis carried out in PSCAD/EMTDC software, which show a good and fast response on reference tracking and disturbance elimination. The correct operation of proposed converter has been approved by simulation and experimental analysis.

The extensive penetration of microgrids in the distribution network poses challenges to the control system, coordination with the network, and especially the traditional current-based protection systems despite many advantages such as reducing power outages, increasing reliability and resiliency, enhancing the flexibility of the power system in supplying loads, and improving the power quality. The main reason for the incorrect performance of current protection schemes is the change in the network fault level due to the connection and disconnection of the distributed generator or the microgrid operating mode change from the grid connected to the islanded and vice versa. To improve the performance of the protection system in the presence of microgrids, some schemes have been presented in two general categories: schemes based on modifying the network behavior and schemes based on modifying the protection system. In the first category, the network behavior during the faults is modified by external equipment to operate the conventional protection schemes properly. In the second category, the protection schemes are modified to correct performance according to the network behavior change during the short circuit faults. Due to the limitations of the schemes in the first category, the schemes of the second ones are more practical. Among the second category schemes, the impedance-based ones can operate in both grid-connected and islanded modes of the microgrid due to their directional nature and independence of the fault level of the network.This article rearranges and reforms the conventional sequence equivalent circuits of the lines during the short circuit faults and presents new equivalent circuits. Also, a protection scheme is proposed using these circuits for low-voltage and medium-voltage overhead and cable lines in smart AC microgrids. The basis of the proposed scheme is the large change in impedance in the proposed delta sequence equivalent circuit. This scheme employs the voltage and current data at the relay location and the magnitude of the positive sequence voltage at the other line end. Therefore, minimum data exchange between two ends of the line and low sampling rate are the features of the proposed scheme. This scheme is independent of the configuration of the microgrid, its operating mode, and uncertainties in the microgrid. Also, it can detect high resistance and high impedance short circuit faults in the grid-connected and islanded mode with a detection time of fewer than two cycles in low voltage lines and about three cycles in medium voltage lines. A case study microgrid is simulated in PSCAD software, and the proposed scheme is implemented on it by MATLAB software. The results show the accurate performance of the proposed protection scheme in detecting short circuit fault types in different conditions.